Application Handbook_Uster_Statistics - Uster Technologies

All USTER ® instruments calculate the confidence intervals automatically and they are part of the testreport. The confidence interval covers the random error component; information on the systematicerror, i.e. instrument tolerances, is provided in our application handbooks.When comparing actual measurements with the data illustrated in the USTER ® STATISTICS, it is ofutmost importance that the total measurement error is kept to an absolute minimum to warrantcompatibility. If this is not the case, false conclusions may be drawn from such a comparison.There are four things that can be done to minimize the measurement error:• proper conditioning under constant standard atmospheric conditions• exact calibration of the instrument• correct settings of the instrument• adequate sample sizeWhen actual measurements are then compared with the USTER ® STATISTICS, they would appear inthe graph as a short vertical line – not as a dot. The top and bottom ends of that line represent theupper and lower limits of the confidence interval with the mean exactly in the middle. We cannoteliminate the random error; however, the confidence interval quickly becomes smaller when thesample size is increased. For detailed information on recommended sample sizes and testingconditions, please refer to chapter 3.In the context of commercial agreements via yarn contracts and product specifications, it frequentlytranspires that disputes result from discrepancies between measurements performed by thepurchaser and by the supplier and from the subsequent comparison of disparate measurements withthe USTER ® STATISTICS. When such incidents are examined more closely, the result often is thatthe basic conditions listed above have been ignored or have simply not been identical in both testinglocations. In other cases, the problem could be quickly resolved by applying the t-test procedure. Itproved that the differences were not statistically significant but strictly random due to a pronouncedsample variability. The t-test procedure along with further detailed explanations is outlined in ourapplication handbooks. A simplified t-test can be performed by comparing the confidence intervals: Ifthe confidence intervals of two mean overlap, then the observed difference between the two means israndom or statistically insignificant; if they are separated, the difference is considered statisticallysignificant. Applying the concept of the confidence interval can be both very helpful and revealing. Itpinpoints the highly variable characteristic of textile materials which should always be taken intoconsideration.USTER ® STATISTICS 34 (36)